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| In [[particle physics]], the '''strong CP problem''' is the puzzling question why [[quantum chromodynamics]] (QCD) does not seem to break the [[CP-symmetry]].
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| According to quantum chromodynamics there could be a violation of CP symmetry in the strong interactions. However, there is no experimentally known violation of the CP-symmetry in strong interactions. As there is no known reason for it to be conserved in QCD specifically, this is a "fine tuning" problem known as the strong CP problem.
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| The strong CP problem is one of the most important [[unsolved problems in physics]].
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| ==What is CP violation==
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| {{Main|CP violation}}
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| CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle (C symmetry), and then left and right were swapped (P symmetry). In [[particle physics]], [[CP violation]] (CP standing for '''Charge Parity''') is a violation of the postulated '''CP-symmetry''' (or '''Charge conjugation Parity symmetry'''): the combination of [[C-symmetry]] ([[charge (physics)|charge]] conjugation symmetry) and [[Parity (physics)|P-symmetry]] (parity symmetry).
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| ==How CP can be violated in QCD==
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| QCD does not violate the CP-symmetry as easily as the [[electroweak theory]]; unlike the electroweak theory in which the gauge fields couple to [[chirality (physics)|chiral]] currents constructed from the [[fermion]]ic fields, the gluons couple to vector currents. Experiments do not indicate any CP violation in the QCD sector. For example, a generic CP violation in the strongly interacting sector would create the [[electric dipole moment]] of the [[neutron]] which would be comparable to 10<sup>−18</sup> [[Elementary charge|e]]·[[Metre|m]] while the experimental upper bound is roughly one trillionth that size.
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| This is a problem because at the end, there are natural terms in the QCD [[Lagrangian]] that are able to break the CP-symmetry.
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| :<math>{\mathcal L} = -\frac{1}{4} F_{\mu\nu}F^{\mu\nu}-\frac{n_f g^2\theta}{32\pi^2}
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| F_{\mu\nu}\tilde F^{\mu\nu}+\bar \psi(i\gamma^\mu D_\mu - m
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| e^{i\theta'\gamma_5})\psi</math>
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| For a nonzero choice of the θ angle and the [[chiral quark]] [[mass phase]] θ′ one expects the CP-symmetry to be violated. One usually assumes that the chiral quark mass phase can be converted to a contribution to the total effective <math>\scriptstyle{\tilde\theta}</math> angle, but it remains to be explained why this angle is extremely small instead of being of order one; the particular value of the θ angle that must be very close to zero (in this case) is an example of a [[fine-tuning|fine-tuning problem]] in physics, and is typically solved by [[physics beyond the Standard Model]].
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| If at least one of the [[quarks]] of the standard model is massless, θ becomes unobservable; i.e. it vanishes from the theory. However, empirical evidence strongly suggests that none of the quarks is massless and so the strong CP problem persists.
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| ==Proposed solutions==
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| There are several proposed solutions to solve the strong CP problem. The most well-known is [[Peccei–Quinn theory]]
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| ,<ref>{{Cite journal
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| |last1= Peccei
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| |first1= Roberto D.
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| |authorlink1= Roberto Peccei
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| |last2= Quinn
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| |first2= Helen R.
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| |authorlink2= Helen Quinn
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| |year= 1977
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| |title= ''CP'' Conservation in the Presence of Pseudoparticles
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| |journal= [[Physical Review Letters]]
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| |volume= 38
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| |pages= 1440
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| |doi= 10.1103/PhysRevLett.38.1440
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| |bibcode= 1977PhRvL..38.1440P
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| |issue= 25
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| }}</ref> involving new [[scalar particle]]s called [[axion]]s.
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| ==References==
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| {{Reflist}}
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| [[Category:Particle physics]]
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| [[Category:Unsolved problems in physics]]
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